Does Driving With The AC On Actually Use More Fuel?

We’ve had this question asked a few times recently and decided to investigate it. Which method of driving is more efficient; driving with your windows down and AC off or driving with your windows up and AC on? Today we find out.

Simple Engine Operation

The battery in your car is a rechargeable battery. When you turn the key in your car to start the engine, the battery uses some of its charge to turn the engine over. It’s very similar to a pull starter on a lawnmower, the only difference is, a car’s starter is automated by an electric motor. In fact, some of the first cars had something much like a lawnmower’s pull starter – a crank in the front that one would turn that help get things going.

Once the engine starts, it’s self-sustaining (as long as there is fuel) thanks to the combustion taking place inside the engine. While the engine is running, the alternator in the car (a small electrical generator) will recharge the battery using some of the power produced by the engine.

If your battery dies thanks to leaving your lights on all night, the electronic motor in the car will not be able to turn the car over and it won’t start up. However, if you were to roll the car down a hill while it’s in gear, the wheels turning will force the pistons to move and the car will start. Once started, the alternator will be able to recharge the dead battery.

Air-Conditioner Operation

When you turn the AC on in your car, it uses energy supplied to it by the alternator. This energy is coming from the engine, which is using the fuel in your gas tank. The AC won’t run properly without the car started because the belt that engages the compressor (which is used to compress the coolant and make it cold) will only run with the engine started. This is the same belt that is responsible for running the alternator and charging your battery.

Which Is More Efficient?

Testing the effect of both methods (windows down, AC off versus windows up and AC on) has been done and debated by several different organizations. One of the leading groups, the Society of Automotive Engineers, conducted a number of tests on the effects of AC versus windows rolled down on automobiles.

The SAE found that running an air conditioner in an automobile decreased gas mileage by 5 to 10%. Stanford University and SAE both recommend driving with your car’s windows down if you are traveling on city streets. However, when on a freeway or highway, the drag produced by your windows being rolled down exceeds the efficiency drop by keeping windows rolled up and AC on. Both organizations recommend that you use your car’s AC when driving at speeds above 45 MPH.

With that in mind, it’s probably best to do your driving in the early morning or later at night when possible. This is when the day is (usually) cooler. Driving with the A/C off and windows up is the most fuel efficient way to operate your automobile.

If you’re really worried about fuel efficiency then work on your braking and accelerating. That’s where most of your gas is spent on.

Patrick

I think you missed that biggest draw on fuel efficiency, the AC compressor. The compressor should only run if you’re cooling your vehicle. This is the largest draw fuel efficiency from AC. If you’re sitting idle, such as at a stop light, you’ll notice your AC compressor turn on/off as evident by the engine RPM dropping/increasing respectively. Any thing that causes the engine RPM to change noticeably is certainly going to impact fuel efficiency.

Steve

The compressor is a load on the engine. The load requires energy for the work. Turning the AC on increases the load thereby increasing the amount of energy needed. The engine works harder burning more fuel.

Matt

Exactly right. They went into a bunch of detail about the battery, etc., but the battery has nothing to do with it. The compressor is engaged through the same belt system as everything else. If the compressor turns on (cooling), then there is a bigger drag on the system overall, reducing it’s efficiency. I always think of it as rolling down a hill in neutral… pretty easy. If you are on a steep hill though, and put your car into first gear… will it still roll? Probably, but not as easily.

Marvin

Technically you cannot say the battery has ‘nothing’ to do with it. Your A/C system utilizes and electric fan to move the air in the cabin. Anytime you use electricity it causes your alternator to work harder which adds ‘drag on the system overall’, as you put it. So, though it is ever so minimal, the battery does have something to do with it.

Zubair

With AC off, my car’s average is 170 km and with the AC on the average is 140 km with a full tank of 8kg.

Doug

If you have a look under the hood while the engine is running and the AC is on, you will notice and hear the compressor kicking on and off. This is activated using a clutch attached to the fan belt, it cycles on and off to maintain the level of coolness of the interior of the car/truck.

Here’s another tidbit: most people don’t realize the windshield defroster is actually like running the a/c, which directs heat to clear the windshield of any fog or ice/snow.
After the windshield is clear, it would be wise to switch off the defroster and send the heat to the floor vents.

Marvin

In most newer cars (meaning anything less than about 30 years old) the defroster function uses a mixture of hot air from your heater system and cold air from your air conditioning system. The reasoning is that you don’t want to shock an ice covered windshield with air that can be well in excess of 140 degrees. Most engines have a water thermostat that ranges from 180 to 195 degrees. If you warmed your car up to operating temperature and then turned the defroster on to a frozen windshield, you could heat it too quickly and possibly crack your windshield. Mixing the cold A/C air with the heater air in your defrost system cools it to a safer temperature preventing damage too your windshield while still offering hot enough air to defrost/deice the windshield.

Austin

The defrost setting utilizes the ac system not to reduce thermal shock, but because the evaporator inside the car withdraws a significant amount of moisture from the air and the windshield is cleared much faster with dry air. Moisture extraction is why there is a drainage built into the evaporator housing, and it’s also why you always see water dripping from a car that is running the AC.

Wade Hawk

“However, if you were to roll the car down a hill while it’s in gear, the wheels turning will force the pistons to move and the car will start.”

This won’t work on a lot of modern cars with electronic ignition and a computer, but works on any car with points and plugs. If battery is fully dead you are SOL trying this method on most modern cars. If you want to know if it will work on your car, park on a hill and unhook the battery. If it works on your car you shouldn’t need the battery, then try and see if you can roll it off.

I found this out the hard way while trying to roll off a 2002 Mitsubishi Mirage with a completely dead battery.

Marvin

A car having points and plugs has nothing to do with it. It is all about having a generator or an alternator. If you unhook your battery, your car will only start if it is equipped with a generator and not an alternator. A generator works with permanent magnets and a coil to produce electricity, an alternator uses electromagnets and a coil to produce electricity. The use of electromagnets requires a small amount of voltage to be applied to function. A completely dead battery or an unhooked battery will not allow the alternator to function. A weak battery that lacks the power to turn your starter motor might still have enough voltage to allow the alternator to function and produce electricity. Also, all gasoline automobile engines have spark plugs. They either have a pointed distributor or electronic ignition, but they all have spark plugs.